I think it has native support for REB. If not, the Librarian software (is it free? I don't remember, I may have paid $15 for it) converts it.
I just like the ability to put absolutely anything I want on it, save PDFs, and even PDFs can be converted with the right OCR software.
The battery life is awesome. Between 8 and 20 hours depending on your preference for screen brightness. I change mine constantly (reading at the bus stop is different than reading in a dark room), so I get on average about 15-16 hours to a charge.
I don't use the modem. I guess I might if I valued hte online bookstore, but most of hte stuff I throw on it is from my local machine. With an extra $15 memory card, I have 128 megs of storage, which is plenty for any sort of vacationing away from my PC.
The best reason to buy one: screen size, screen size, screen size.
The worst reason: what if your battery breaks, like pitcher's? Doesn't seem like Fictionwise really has a hardware support infrastructure setup.